1. Field of the Invention
The present invention relates to an image processing method and an image processing apparatus, and particularly to a process for determining a conversion relationship of a table or the like relating to data generation for color materials such as ink, which include a plurality of color materials of the same color but different in concentration and used in an image output apparatus such as an ink jet printer.
2. Description of the Related Art
In general, when an image output apparatus such as a printer is used to print and output a color image, four color materials for Y (Yellow), M (Magenta), C (Cyan), and K (black) are used. For example, an ink jet printer uses Y, M, C, and K inks. Further, a copy machine or a printer of an electrophotography uses Y, M, C, and K toners.
A color space realized by these color materials, used in the image output apparatus, is normally different from a color space for an original image data. Thus, in the field of image processing in which color material data is generated on based on image data, various image processing methods have been proposed to reproduce faithfully colors or gradations shown by the original image data or to reproduce desired colors and the like.
Among such image processing methods, the one shown in FIG. 25 is known. This image processing configuration has, as shown in FIG. 25, a luminance density converting section 1501, a UCR/BG processing section 1502, a BG amount setting section 1503, and a UCR amount setting section 1504. The luminance density converting section 1501 converts 8 bit data R′G′B′, which is inputted luminance information, into CMY on the basis of the following equations:C=−α log(R′/255)  (1)M=−α log(G′/255)  (2)Y=−α log(B′/255)  (3)                where, α is a predetermined real number.        
Then, the UCR/BG processing section 1502 uses β(Min (C, M, Y), μ), set in the BG amount setting section 1503, and the value μ%, set in the UCR amount setting section 1504, to convert the CMY data as shown by the following equations:C′=C−(μ/100)×Min(C, M, Y)  (4)M′=M−(μ/100)×Min(C, M, Y)  (5)Y′=Y−(μ/100)×Min(C, M, Y)  (6) K′=β(Min(C, M, Y), μ)×(μ/100)×Min(C, M, Y)  (7)                where β(Min (C, M, Y), μ) is a real number varying depending on Min (C, M, Y) and μ. This value enables the setting of a manner of adding K ink. The UCR amount and the BG amount are important parameters for a color separating process executed for a color printer, because these amounts have much effect on the color reproduction range of the color printer and the granularity of the printer associated with the manner of adding K ink (Indian ink).        
Thus, depending upon the setting of β, for example, a color with the lowest density may be prevented from having a value of 0 without replacing any of Y, M, or C with K, and a combination of Y, M, C, and K which does not contain a value of 0 may be provided. Generation of such data enables outputting of images without any decrease in saturation or density.
The above described image processing relating to the color transformation is commonly carried out using a lookup table (LUT) having data used to carry out the color transformation as well as interpolation. The contents of the table are determined by printing patches for a plurality of combinations of the color materials Y, M, C, and K and subjecting the patches to a colorimetric operation. Specifically, if signal values for the respective color materials Y, M, C, and K constituting each patch data are expressed by 8 bits, then for example, patches are printed for 4,096 colors obtained by combining signal values of respective four color materials, each of which is one of 0, 32, 64, . . . , 223, and 255 that are obtained by equally dividing 256 values for the respective colors into eight parts. Then, the patches are subjected to a colorimetric operation, and on the basis of the results of the colorimetric operation, a combination of Y, M, C, and K that reproduces a predetermined target is determined to be table data.
Further, ink jet printers and the like have been required to provide color printing with higher quality image. To achieve this, it has been common to use a plurality of inks of the same color such as magenta or cyan but different in concentration of stuff such as dyes in the color material. This system enables the sense of granularity associated with ink dots to be reduced in a low density portion of a printed image such as its high light portion.
However, even if image processing is executed so as to improve the reproducibility of the image output apparatus such as a printer as described above, actual printing in the printer or the like becomes improper owing to the relationship between the color materials and printing sheets. As a result, images cannot be reproduced as desired.
The process shown in FIG. 25 basically enables four color materials Y, M, C, and K to be simultaneously used. This tends to increase the total amount of used color materials. In particular, when a color with a high saturation is to be reproduced at a low lightness, the respective color materials realizing the color have high signal values, and then the amount of each color material used increases. Further, when a plurality of inks of the same color but different in concentration are used, the total amount of ink used increases.
In this case, some printers do not particularly restrict, for example, the use of ink or toner at a darkest point. For example, if the amount of cyan used at its maximum output is assumed to be 100%, then at the darkest point, the total amount of the four color materials used, that is, a total color material use amount is 400%. Thus, in actual printing, an ink jet printer cannot sufficiently absorb ink depending on the ink absorption characteristics of print sheets, and then, ink may overflow or bleed to hinder the density or the like from being accurately realized. Further, an electro-photographic printer using toner cannot sufficiently fix the toner to print sheets depending on the toner adhesion characteristics of the sheets. Again, the density cannot be accurately realized.
This problem may occur in the case of generating data for patches and printing them. If the patches are thus not accurately printed, colorimetric values, as a matter of course, do not reflect the printing characteristics of a printer that has printed the patches. This makes it impossible to generate the table for accurate color transformation.